1. Chapter – 8 Fluorescence
Raditionless Transitions
Radiation Transitions
Fluorescence

2. Fluorescence Sir George Gabriel Stokes, 1st Baronet
Mathematician and Physicist. Worked at Cambridge.
In 1852, in his famous paper on the change of
Wavelength of light, he described the phenomenon of Fluorescence, as exhibited by Fluorspar and Uranium Glass.
Stokes Shift
Low-intensity lighting and haze in a concert hall
allows laser effects to be visible
Fluorescent minerals

4. Raditionless Transitions

7. How to measure fluorescence lifetime ?
A streak camera (ultra fast high speed detector) is an instrument for measuring the variation in a pulse of light's intensity with time.
Molecules are excited by a very short pulse (close to a d-pulse) at t = 0 and the decay of florescence intensity is measured. Usually by Time Correlated Single Photon Counting (TCSPC).

8. Excited-State Lifetimes of Atoms
The excited state of an atom will have an intrinsic lifetime due to radiative decay given by:
Nj(t) = Nj(0) exp (-t/)
 is the radiative lifetime defined as 1/ Aji.
The above expressions give the radiative lifetime
Spontaneous or induced emission usually occurs shortly
after the system is promoted to the excited state, returning the system to a state with lower energy.
This return to a lower energy level is often loosely described as decay and is the inverse of excitation.

12. Life Time of Excited State

13. We have used the following expressions
nj(t) = nj(t=0) exp(- Aji t) for fluorescence emission
Nt = N0 exp ( -  t) for radioactive decay

14. Host-Guest System

18. Atomic-Absorption Spectroscopy
Light source – Lasers, Xe-Lamp, Halogen lamp
Test Sample
Detector - Monochromators, Photomultiplier tubes
Recorder

19. Applications Optical Properties of Semiconductor
Optical Measurements in semiconductors are useful:
To study the band structures of semiconductors
To determine the energy bandgap
Free carrier
Used to study lattice vibrations.

20. Absorption spectrum of a film of propyl gallate 40 nm thick
on a silica substrate
Band gap = 1238/ (nm)
= 1238/ 340
= eV
340 nm
Absorption edge

22. C.B. = Conduction Band

24. Fluorescence Spectrometer

25. Light Source : Spectral output over wide range
Tungsten halogen lamp, Mercury Lamp,
Xenon Lamp
Wavelength Selection: Interference Filter ,
Monochromators
Detectors: Photomultiplier Tube
Read out Devices: Digital Display
Sample Holder: (i) Normal Incidence.
Fluorescence given off in directions.
Normally collected from the front surface
of the cell at Right angle to the incident
beam.
(ii) in-line with the incident beam